Analysis of Anomalous Tropical Cyclone Activities over the Western North Pacific and the South China Sea in 2010

The possible reasons for anomalous tropical cyclone (TC) activity over the western North Pacific (WNP) and South China Sea (SCS) were studied, and the possible effects of dynamic conditions and the thermal state on the frequency of TC genesis and landfall, derived from circulation patterns and boundary forcing, were analyzed using TC data from the China Meteorological Administration, NCEP/NACR reanalysis data, and the outgoing long-wave radiation data from the US NOAA. The results showed that TC genesis was much less frequent than normal but that TC landfall was near normal in 2010. The TC genesis locations were west of the usual genesis longitudes. The circulation patterns leading to fewer TCs were characterized as strong western North Pacific subtropical highs and South Asia highs, westward monsoon troughs, and vertical wind shear, anticyclone anomalies at low and high levels of the troposphere over Lake Baikal. Each of these features does not favor the genesis of TC in the eastern part of the WNP. Boundary forcing showed that the tropical Pacific experienced an El Niño event that ended in the late spring and early summer 2010 while a new La Niña event formed that July. From July to October, there was a positive sea surface temperature (SST) anomaly over the WNP west of 140�E while convection was suppressed east of 140�E. The subsurface temperature over the warm pool was much higher than normal, corresponding to an 850 hPa easterly wind anomaly. This thermal state led to the monsoon trough moving west of its average position and TC genesis in the eastern WNP. The SST and subsurface temperature features in the warm pool were the key thermal factors in the lower frequency of TCs in the western WNP in 2010. Further analysis showed that there was an anticyclone 500 hPa wind anomaly over the middle latitudes of the North Pacific from July to October 2010. The easterly anomaly to the south of the anticyclone was accompanied by another in the west of the WNP and SCS. The easterly anomaly extended southward to 25�N and westward to mainland China and the western anomaly extended northward to 15�N. Such a wind anomaly pattern favors the westward movement of TCs and then making landfall in mainland China. These are, therefore, the circulation conditions required for much less frequent TC genesis and almost normal TC landfall.